1. Field of the Invention
The present invention relates to an ethylene homopolymer and its molded article. More precisely, the invention relates to an ethylene homopolymer having a high melting point relative to its density, and having good low-temperature heat-sealability and moldability, and also to its molded article.
2. Description of the Related Art
As a rule, polyethylenes from ethylene through polymerization are grouped into low-density polyethylene (LDPE) to be from ethylene through high-pressure radical polymerization and high-density polyethylene (HDPE) also to be from it but through polymerization in the presence of a Ziegler-Natta catalyst, depending on the methods of producing them. Polyethylenes of those types have different characteristics, and are used for different applications depending on their characteristics.
Specifically, low-density polyethylene (LDPE) to be produced through high-pressure radical polymerization of ethylene has a combined structure of long-chain branches and short-chain branches, of which the melt tension is high and the activation energy for melt fluidity is large. Therefore, it is suitable to high-speed molding and is favorable to applications for films and blow molded containers. However, as having a low melting point relative to its density, LDPE is defective in that its heat resistance is poor. In addition, since its molecular weight distribution is broad and since it contains a low-molecular-weight, low-melting-point component (low polymer), LDPE is still defective in that its environmental stress crack resistance (ESCR) is poor and its mechanical strength including, for example, impact strength, tensile strength and tear strength is low.
On the other hand, high-density polyethylene (HDPE) to be produced through polymerization of ethylene in the presence of a Ziegler-Natta catalyst has good heat resistance and high mechanical strength, as its structure has few branches. However, since its melt tension is low and its activation energy for melt fluidity is small, HDPE is defective in that its high-speed moldability is poor. In addition, HDPE is further defective in that its heat-sealing temperature is high. In that situation, improving such LDPE and HDPE is desired.
Heretofore, nobody has obtained ethylene homopolymer having well-balanced properties of good heat resistance, high mechanical strength, good low-temperature heat-sealability and good moldability.
At present, linear low-density polyethylene (LLDPE) to be produced through copolymerization of ethylene and .alpha.-olefins in the presence of a Ziegler-Natta catalyst are used in place of ethylene homopolymer. However, since the ability of the Ziegler-Nata catalyst to catalyze the copolymerization is poor, the compositional distribution in the resulting linear low-density polyethylene (LLDPE) is broad. Therefore, LLDPE is problematic in that its molded articles such as films and others are often sticky. Another problem with it is that LLDPE shall contain a larger amount of a low-molecular-weight, low-melting-point component (low polymer) with its density being lowered, and its molded articles could not have high mechanical strength.
In addition, linear low-density polyethylene (LLDPE) is often limited in its production, as it must be produced at high temperatures or must be produced through multi-stage polymerization.
Apart from the above, blending various polyolefin resins such as HDPE, LDPE, LLDPE and others is tried for improving the properties of the resins. However, this is still defective, as requiring some additional equipment for blending them, and taking a lot of time for obtaining the intended resins having high mechanical strength and good moldability.
Given that situation, if an ethylene homopolymer having well balanced properties of good heat resistance, high mechanical strength, good low-temperature heat-sealability and good moldability could be obtained, it could solve all the problems noted above.